**10. M-X/N/NCS/CN…X systems**

Simple metal bound ligands, capable of donating electrons, can also be used as XB acceptors in the construction of supramolecular structures.[44] Metal complexes are of particular interest because of the possibilities of using halogen bonds as a tool in the modification of the redox, magnetic, optical, and chemical reactivity of the metal complexes.[44–46]

The palladium pincer complex {2,6-bis[(di-t-butylphosphino)methyl]-phenyl}palladium (PCPPd) halides, PCPPdX (X=Cl, Br, or I) have been studied by Johnson and Rissanen as the XB acceptor in systems with I2 as XB donor.[47] The all three crystal structures have similar basic features (Fig. 15). However, the halogen bond strength was found to increase in the order Cl ‹ Br ‹ I, suggesting that the XB interactions are mainly electrostatic as expected.[47] The other intermolecular contacts were relatively weak. It is, however, worth mentioning that in PCPPdI. I2 the Pd…I-I…π interaction also appeared to result in the formation of a chain-like structure. This was not observed in the other two structures.

154 Recent Advances in Crystallography

triiodoimidazol.‡

closely related electrostatic interactions.

**10. M-X/N/NCS/CN…X systems** 

**Figure 14.** Hydrogen and halogen bonds in the co-crystal of 1,2-diiodoimidazol and 1,3,4-

Halogen bonds involving selenium as the XB acceptor have not been widely studied.[29] The chain structure formed by connecting di-ter-butyliodophosphane selenide molecules through Se…I halogen bonding is one example of such a system.[29] In this structure the Se…I distance was found to be only slightly shorter than the sum of the van der Waals radii of selenium and iodine. Another example of Se…I interaction can be found in the crystal structure of iodoisopropylphosphane selenide.[29] In both structures, selenium is also involved in Se…H hydrogen bonds, providing another example of the interplay between the

Simple metal bound ligands, capable of donating electrons, can also be used as XB acceptors in the construction of supramolecular structures.[44] Metal complexes are of particular interest because of the possibilities of using halogen bonds as a tool in the modification of

The palladium pincer complex {2,6-bis[(di-t-butylphosphino)methyl]-phenyl}palladium (PCPPd) halides, PCPPdX (X=Cl, Br, or I) have been studied by Johnson and Rissanen as the XB acceptor in systems with I2 as XB donor.[47] The all three crystal structures have similar basic features (Fig. 15). However, the halogen bond strength was found to increase in the

the redox, magnetic, optical, and chemical reactivity of the metal complexes.[44–46]

**Figure 15.** Crystal structures with halogen bonds in PCPPdX…I2 ( X=Cl, Br, or I). (a) PCPPdCl…I interactions, (b) PCPPdBr…I interactions, (c) PCPPdI…I interactions.[47]

The bonding preferences of halogen and hydrogen bonds can also be found among the assemblies of metal complexes. The co-crystals of [RuI2(H2dcbpy)(CO)2] (H2dcbpy = 4,4' dicarboxylic acid-2,2'-bipyridine), I2 and methanol is an example of such system (Fig. 16).[48] The two [RuI2(H2dcbpy)(CO)2] complexes are held together strongly by the hydrogen bonds between the carboxylic acid groups. The iodide ligands bonded to the ruthenium centers are involved only in halogen bond, thus extending the structure into a chain of metal complexes. The halide ligands are linked by halogen bonds through two I2 molecules. It is worth noticing that the halogen bonds between the I2 molecules are bifurcated, and the solvent molecule is supporting the structure via hydrogen bond. The I-I…I bond angle was nearly linear (167°) for the first I2, but due to the bifurcated nature only 137.8° for the second I…I2 contact, which differs from the conventional XB bond angles.

**Figure 16.** Halogen and hydrogen bonds in the structure of [RuI2(H2dcbpy)(CO)2]·I2.[48]

Any ligand possessing a free electron pair can be seen as a potential halogen bond acceptor. The N-bound thiocyanate in *cis*-diisothiocyanato-bis(2,2'-bipyridyl-4,4' dicarboxylato)ruthenium(II) provides an example of such a ligand. The structure of the [RuI2(H2dcbpy)(CO)2]·2I2(Fig. 17) adduct have been obtained at room temperature by mixing I2 and the complex in methanol.[49] In this structure, the sulfur atom of one of the thiocyanate ligands forms bifurcated halogen bonds with two I2 molecules. Based on the distances, these bifurcated bonds are weaker than the non-bifurcated one.

**Figure 17.** Halogen bonding interactions in [RuI2(H2dcbpy)(CO)2]·2I2.[49]

Ormond-Prout, Smart, and Brammer proposed that halogen bonds can be used to predict and control the process of self-assembly and to fine-tune the electronic properties of cyanometallates.[50] To confirm this assumption they synthesized two types of halopyridium hexacyanometallate salts, (3-XpyMe)3[M(CN)6] and (3, 5-X2pyMe)3[M(CN)6] (X=Cr, Fe, Co).[50] The authors harvested a total of ten crystals, and found out that five out of each family of compounds were isostructural, while other structures were the solvates, (3- IpyMe)3[Fe(CN)6]·2MeCN(2·2MeCN) and (3,5-Br2pyMe)3[Cr(Cr(CN)6]·(10·4H2O). The halogen bonding distances in these structures were shorter than the sum of the van der Waals radii. In the case of (3,5-Br2pyMe)3[Cr(Cr(CN)6]·(10·4H2O), a weak additional C-Br…O halogen bond was found, which can be attributed to the competition between the halogen bonding and the O-H…N hydrogen bonding. The close-to-linear geometry of the CN…X halogen bonds found in the all structures suggests that these interactions predominantly involved the exo lone pair of nitrogen atom. However, the structures contained another type of halogen bonds with less linear CN…X contacts (CN…X ‹ 105°). Such angles indicate that the triple bond between the C and N contributes to the halogen bonding interaction leading to X…π contact (Fig. 18). In this series the strength of the halogen bonds was found to be dependent on the metal center (Cr ‹ Fe ‹ Co). This is a good example of how the metal center can be used for modification of halogen bonds. In this particular example, the primary reason for the different behavior of the different metals has been attributed to the metalcyanide π-back-donation.[50]

**Figure 18.** Halogen bonding contacts in (3-IpyMe)3[Fe(CN)6]·2MeCN.[50]
